Pyrylium salts as pickling inhibitors



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3,434,973 Patented Mar. 25, "1969 This invention relates to pyryliumsalts as picking inhibitors for iron and steel. The invention alsopertains to pickling compositions containing the aforesaid pyryliumsalts.

In the pickling art various materials, commonly organic compounds, aredispersed in the acid cleaning solution in order to retard corrosion ofthe underlying metal while at the same time not materially afiecting therate of attack on the scale. Inhibitors used in sulfuric acid picklingbaths reduce metal attack substantially but not completely therebypermitting suliicient hydrogen evolution to effect dislodging of thescale and rust from the surface. In hydrochloric acid pickling and inall acid cleaning applications for that matter maximum inhibition ofmetal dissolution commensurate with reasonably effective scale removalis always a desideratum.

The theory generally advanced to explain the action of picklinginhibitors supposes formation of an adsorbed or chemisorbed layer on thenewly bared metal surfaces thereby preventing attack -by the acid bath.

With a view to obtaining improvements in pickling inhibitors we havediscovered that certain pyrylium salts are particularly effective inthis capacity and the provision of such materials and their use aspickling agents con-= stitutes the principal objects and purposes of theinvention. Other objects and purposes will become apparent subsequently.

'Pyrylium salts are known chemical entities the description andpreparation of which are set forth in the chemical and technicalliterature. An especially detailed account on their synthesis can befound in U.S. Patent 3,250,615.

In practicing the invention we have achieved excellent corrosioninhibition when using triarylpyrylium salts wherein the structure admitsof a planar configuration. Accordingly the triarylpyrylium salt shouldbe free of large or bulky groups which might prevent the molecule fromassuming a planar arrangement. It is our opinion that such substituentshinder adsorption of the inhibitor to the freshly exposed metal surface.

Triphenylpyrylium tetrachloroferrate is an example of a preferredarylpyrylium salt. It can be obtained by the reaction of benzaldehydewith acetophenone using an acidic condensing agent. For details of thepreparation reference is made to the aforecited US. patent.

The 2,4,6-triphenylpyrylium tetrachloroferrate is unusual in that itprovides practical inhibition at rem-arkably low concentrations. In factit affords excellent protection when formulated in a pickling bath atconcentrations as low as 0.02% by weight or less. The practical rangefor the additive is from about 0.01% to about 0.02%. It is effective inboth sulfuric and hydrochloric acid, the two most commonly used picklingbath acids.

In using the 2,4,6-triphenylpyrylium tetrachloroferrate it isconveniently employed in the form of a concentrate formulationcontaining from about 10 to 20% of the inhibitor, about 20% of adetergent such as a alkyl-phenolpolyether-alcohol, about 1 to 2% ofconcentrated hydrochloric acid and about 69 to 58% water. A typicalformulation is as follows in which the components are on a weightpercent basis:

Addition of one part of the concentrate to 1,000 parts of the picklingbath provides a suitable concentration for pickling inhibition.

The detergents used in pickling baths are not particularly critical andany number of types and kinds can be used. Preferred species are thealkylaryl-polyether= alcohols although sodium alkylarylsulfonates alsoperform reasonably satisfactorily.

Pickling baths containing one part of the aforedescribed concentrate to1,000 parts by weight of the pickling bath showed 80.6% inhibition ofacid attack on iron in 30% HCl at 40 C. and 85.6% in 20% sulfuric acidat 60 C.

What is claimed is: e

1. A pickling bath composition suitable for use with iron and steelconsisting essentially of an aqueous solution of acid selected from theclass consisting of sulfuric and hydrochloric and as a picklinginhibitor 2,4,6-triphenylpyrylium tetrachloroferrate in theconcentration of 0.01% about 0.02% by weight.

2. A method of inhibiting acid attack in steel and iron pickling bathswhich comprises exposing the fresh metal surface to the composition ofclaim 1.

3. An inhibitor concentrate formulation consisting essentially of2,4,6-triphenylpyrylium tetrachloroferrate of the following ingredientsby weight:

Percent 2,4,6-triphenylpyrylium tetrachloroferrate 10-20Nonylphenol-polyether-alcohol 20-20 Concentrated hydrochloric acid (37%)l-2 Water 69-58 References Cited UNITED STATES PATENTS 1; United StatesPatent ()jfi

1. A PICKLING BATH COMPOSITION SUITABLE FOR USE WITH IRON AND STEELCONSISTING ESSENTIALLY OF AN AQUEOUS SOLULTION OF ACID SELECTED FROM THECLASS CONSISTING OF SULFURIC AND HYDROCHLORIC AND AS A PICKLINGINHIBITOR 2,4,6-TRIPHENYLPYRYLIUM TETRACHLOROFERRATE IN THECONCENTRATION OF 0.01% ABOUT 0.02% BY WEIGHT.